The present invention relates to the general field of turbomachines. The present invention relates in particular to taking account of thermal unbalance when starting a turbomachine.
Thermal unbalance is a temporary unbalance that occurs in the rotor of a turbomachine after it has been stopped. Since hot air is lighter than cold air, temperature nonuniformities occur in the air-flow passage, which deform the rotor by a bimetallic effect. The thermal unbalance may be one hundred times greater than the residual unbalance of the rotor.
However, a turbomachine must pass through one or more critical speeds before reaching its operating speed. Thus, while restarting the turbomachine, on passing through critical speeds the thermal unbalance can give rise to strong vibration, which can degrade the reliability of the turbomachine. Thermal unbalance may also have the consequence of contact being made between the stator and the rotor, which can deteriorate the integrity of the components involved.
There are various known solutions that seek to reduce the undesirable effects of thermal unbalance:                not restarting the turbomachine for a certain length of time after it has stopped;        specific procedures performed while the engine is stopped or before it is restarted, for the purpose of reducing temperature nonuniformity (ventilation, successive rotations of the engine, injecting a stream of air into the air-flow passage, . . . );        enlarging clearances between the stator and the rotor to prevent them making contact while restarting the turbomachine; and        modifying the architecture of the engine in order to limit the effects of thermal unbalance.        
Nevertheless, those known solutions present certain drawbacks:                not restarting for a certain length of time penalizes the availability of the turbomachine;        specific procedures require action to be taken by an operator, e.g. by the pilot for an aeroengine;        enlarging clearances between the stator and the rotor is detrimental to the performance of the turbomachine; and        adapting the architecture of the engine is detrimental to the weight of the turbomachine.        
For example, Document FR 2 185 753 describes injecting compressed air into the air-flow passage, via a channel provided for that purpose. That document also mentions other solutions, in particular using turning gear to put the turbomachine into rotation before restarting it. That particular solution involves action being taken by an operator before restarting.
There therefore exists a need for a solution that is more effective when starting a turbomachine that might be suffering from thermal unbalance.